Turbulent flow study of an isothermal diesel spray injected by a common rail system

Abstract

The flow characteristics of the intermittent spray of a single-hole diesel nozzle ( d o=0.11 mm) having a 1-spring holder, used in the injection system of heavy-duty diesel engines, were experimentally investigated. The hole belongs to a mini-sac 5-hole nozzle where only on hole is drilled. The mean velocity and turbulent characteristics of the diesel spray injected intermittently by a Common-Rail (CR) system into a pressurized vessel at room temperature were measured by using a 1-D PDPA (phase Doppler particle analyzer). The injection duration was a little stretched out (3 ms) to increase the quasi-steady part of the spray. The axial velocity of the droplets was studied in the main parts of the single-hole nozzle spray, i.e. the leading edge; the central part and the trailing edge. Temporal distributions of the mean axial velocity and its rms were constant in the central spray part, and they showed peaks in the leading edge of the spray. The radial distribution of the normalized axial mean velocity was similar to that of the free gas jet within r/ r0.5=1.0−1.5 regardless of time, which is consistent with the theoretical velocity distributions suggested by Hinze. However, in the leading edge near the centreline axis, the normalized axial mean velocity displayed higher values. The turbulence intensity of the axial velocity measured along the radial direction was similar to the free gas jet within r/ r0.5=0.5 and higher beyond. However, the turbulence intensity in the leading edge was higher than in the leading edge and the central part within r/ r0.5=0.7 where it showed values of the 40–60% of the local mean velocity. The factors of skewness and flatness approached to those of the free gas jet in the central part and the trailing edge. In the leading edge, the flatness factor has presented dispersed values, and the skewness factor was always higher than were those of the two other parts of the spray. The gradient of the half-width exhibited a linear decrease with time since the beginning of the injection to reach the value of 0.106 at the end of the injection. The virtual origin value was within 10–13 mm independently of the injection pressure, and the spray cone angle, determined in comparison to the virtual origin, was close to 30°. The axial decrease of the mean axial velocity showed a great similarity with that of the free gas jet in the central spray part. However, the axial decrease of the rms-velocity was faster than that of the free gas jet.